A bistable optical device has been previously described in U.S. patent application, Ser. No. 042,411, of H. S. Hinton, A. L. Lentine, and D. A. B. Miller, entitled "Symmetric Optical Device", that includes a pair of p-i-n photodiodes with quantum-wells in the intrinsic regions. This device operates as a bistable memory element with complementary and symmetric optical outputs, thereby functioning as an optical S-R latch with well-known set and reset states. The device exhibits time sequential gain in that the state of the device can be either set or reset with low power level beams and that the state of the device can be subsequently determined with high power level beams. Another feature of the device is that the switching point is determined by the ratio of the two input light beams, thus making the device insensitive to optical power supply variations when both of the two input beams are derived from the same source. The complementary output beams facilitate use in well-known "dual-rail" logic applications.
However, applications may arise where it is desirable to have a three-state optical device.